Myosins are actin activated ATPases capable of supporting contractile and motile activities. Many classes of myosins exist, the best studied being the muscle or conventional form. Another class is myosin I. Class I myosins have been shown to be membrane and/or vesicle associated and have roles both in cell motility and organelle transport. We have cloned and sequenced a myosin I complementary DNA (cDNA) from Aspergillus nidulans and have designated its gene myoA. A myoA null mutant has been made by gene disruption in a diploid strain of A. nidulans. We have been unable to recover viable haploids from this diploid. This suggests that myoA is an essential gene in A. nidulans. We have constructed a conditionally lethal mutant myoA strain using the inducible promoter from the alcA alcohol dehydrogenase gene of A. nidulans. These strains are only viable in the presence of inducer and are dead when grown on repressing, glucose containing, medium. Microscopic examination of the conditionally lethal mutants in the presence of glucose has shown that they have abnormal hyphal structure, suggestive of a failure in vectoral transport. This is the first demonstration that a myosin I gene is essential and provides us with a genetic approach to the study of myosin I function in A. nidulans. Immunolocalization by fluorescence microscopy has determined that MYOA protein is enriched at the growing hyphal tip, consistent with a role in polar vesicular transport. We are currently trying to determine the essential intracellular function of MYOA. The requirement for myoA function, the limited complexity of the myosin gene family combined with the structural organization of A. nidulans, its sophisticated genetics and the means to manipulate its genome using molecular genetics makes it a system well suited for the study of myosin gene function. This is the only system established thus far that allows us to examine myosin I functions using a genetic approach.